[PATCH 13/14] pwm: meson: add support PWM_POLARITY_INVERSED when disabling

[Martin Blumenstingl <martin.blumenstingl@googlemail.com>]

meson_pwm_apply() has to consider the PWM polarity when disabling the
output.
With enabled=false and polarity=PWM_POLARITY_NORMAL the output needs to
be LOW. The driver already supports this.
With enabled=false and polarity=PWM_POLARITY_INVERSED the output needs
to be HIGH. Implement this in the driver by internally enabling the
output with the same settings that we already use for "period == duty".

This fixes a PWM API violation which expects that the driver honors the
polarity also for enabled=false. Due to the IP block not supporting this
natively we only get "an as close as possible" to 100% HIGH signal (in
my test setup with input clock of 24MHz and measuring the output with a
logic analyzer at 24MHz sampling rate I got a duty cycle of 99.998475%
on a Khadas VIM).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
---
 drivers/pwm/pwm-meson.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
index 900d362ec3c9..bb48ba85f756 100644
--- a/drivers/pwm/pwm-meson.c
+++ b/drivers/pwm/pwm-meson.c
@@ -245,6 +245,7 @@ static void meson_pwm_disable(struct meson_pwm *meson, struct pwm_device *pwm)
 static int meson_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 			   struct pwm_state *state)
 {
+	struct meson_pwm_channel *channel = pwm_get_chip_data(pwm);
 	struct meson_pwm *meson = to_meson_pwm(chip);
 	int err = 0;
 
@@ -252,7 +253,27 @@ static int meson_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 		return -EINVAL;
 
 	if (!state->enabled) {
-		meson_pwm_disable(meson, pwm);
+		if (state->polarity == PWM_POLARITY_INVERSED) {
+			/*
+			 * This IP block revision doesn't have an "always high"
+			 * setting which we can use for "inverted disabled".
+			 * Instead we achieve this using the same settings
+			 * that we use a pre_div of 0 (to get the shortest
+			 * possible duration for one "count") and
+			 * "period == duty_cycle". This results in a signal
+			 * which is LOW for one "count", while being HIGH for
+			 * the rest of the (so the signal is HIGH for slightly
+			 * less than 100% of the period, but this is the best
+			 * we can achieve).
+			 */
+			channel->pre_div = 0;
+			channel->hi = ~0;
+			channel->lo = 0;
+
+			meson_pwm_enable(meson, pwm);
+		} else {
+			meson_pwm_disable(meson, pwm);
+		}
 	} else {
 		err = meson_pwm_calc(meson, pwm, state);
 		if (err < 0)
-- 
2.21.0